Heretofore, as a glass substrate for an LCD panel, an alkali-free glass containing no alkali metal oxide has been used. The reason is that if an alkali metal oxide is contained in a glass substrate, alkali ions in the glass plate will be diffused into a semiconductor film of a thin film transistor (TFT) to be used for driving an LCD panel during heat treatment carried out in a process for producing an LCD panel, thus leading to deterioration of the TFT characteristics.
Further, since an alkali-free glass has a low coefficient of thermal expansion (30×10−7 to 60×10−7/° C.) and a high strain point, its change in dimensions in a process for producing an LCD panel is small, and the influence over the display quality by thermal stress at the time of use of the LCD panel is small, and accordingly it is preferred as a glass substrate for an LCD panel.
However, an alkali-free glass has the following drawbacks in view of production.
An alkali-free glass has very high viscosity and is hardly molten, and its production involves technical difficulty.
Further, in general, only a poor effect of a refining agent can be produced for an alkali-free glass. For example, in a case where SO3 is used as the refining agent, since the temperature at which SO3 is decomposed and released as bubbles is lower than the glass melting temperature, the most part of SO3 added is decomposed and volatilized from the molten glass before refining is carried out, and no sufficient refining effect will be achieved.
In recent development of technology, use of an alkali glass substrate containing an alkali metal oxide as the glass substrate for an LCD panel begins to be studied (Patent Documents 1 and 2), since it becomes possible to carry out heat treatment in a process for producing a TFT panel at a relatively low temperature (about 250 to 300° C.) which has been carried out at from 350 to 450° C.
Since glass containing an alkali metal oxide generally has a high coefficient of thermal expansion, in order that it has a coefficient of thermal expansion (30×10−7 to 60×10−7/° C.) preferred as a glass substrate for an LCD panel, B2O3 having an effect to lower the coefficient of thermal expansion is usually contained (Patent Documents 1 and 2).
However, in the case of a glass composition containing B2O3, B2O3 is volatilized when the glass is melted particularly in a melting step and in a refining step, whereby the glass composition tends to be inhomogeneous. If the glass composition is inhomogeneous, flatness when the glass is formed into a plate shape tends to be influenced. A glass substrate for an LCD panel is required to have high flatness so as to keep a constant distance between two sheets of glass sandwiching a liquid crystal i.e. cell gap, so as to secure the display quality. Accordingly, in order to secure a predetermined flatness, after glass is formed into plate glass by float process, the surface of the plate glass is polished. If no desired flatness of the plate glass after forming is achieved, the time required for the polishing step will be long, thus lowering the productivity. Further, considering the environmental burden by volatilization of B2O3, the content of B2O3 in the molten glass is preferably lower.
However, if the B2O3 content is low, it has been difficult to lower the coefficient of thermal expansion. Further, it has also been difficult to obtain a predetermined strain point, etc. while the increase in the viscosity is suppressed.    Patent Document 1: JP-A-2006-137631    Patent Document 2: JP-A-2006-169028